1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films over a substrate having an insulating surface has been attracting attention. Such transistors are used in a wide range of electronic appliances such as an integrated circuit (IC) and an image display device (display device).
As a material having semiconductor characteristics suitable for transistors, a metal oxide has attracted attention, and transistors in which a channel formation region is formed using such metal oxide having semiconductor characteristics are known (see Patent Documents 1 and 2).